Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Kog'maw is a very versatile champion.
I personally play an AP Kog'maw over AD because i've seen it to me so much more effective especially when protection towers. What I mean by this is when champions tend to clump for a tower push you can use Void Ooze and Living Artillery to own people from a far. The more clumped they stay the more ownage he becomes.
Few things you should know:
He is very squishy in beginging.
He can be very powerful if fed in early game.
When pared with a tank or a person who can stun he is very destructive.
He passive Icathian Surprise can be very useful when fighting multiple champs if they are all low health because you can suicide kill them all ;D. (Personal Fan Favorite)
Caustic Spittle is nice if you're facing tanks, but if you have a tank or a stun in lane with you don't worry about it till end game.
Void Ooze is a mana soak so use it carefully.
Clarity is always nice especially if you're solo laning or are very accurate with Void Ooze and Living Artillery like me.
I use Ghost, Clarity because its nice chasing down champs and having that last mana switch if needed.
Cleanse is nice also for those annoying champs.
Heal is useless period never get it with Kog, if you have that much trouble staying alive with him pick a diff Champ.
Kog'maw can be a very supportive Champ or a very Abusive Champ, i tend to be more supportive then abusive given the circumstances. Void Ooze is nice when running away from the enemies especially if they aren't that smart and run through the entirety of it. Bio-Arcane Barrage is very nice because you have that super range for those enemies who think they can get away, but really they have no chance. haha.
The items i have selected are as far as i've gotten in game. My team usually wins or loses before i get anymore. If your game should last longer than those items aim, for Zhonya's Ring or Abyssal Scepter depending on the team you are facing. Abyssal Scepter is nice for the magic resist and high AP. Also for some reason the masteries was not working so you can't see it exactly.
Hope this is of some use let me know your opinions, but don't be too harsh!